Advance curves for different engines

[salty_monk]

This has come up a few times but we haven't had a thread about it... it interests me.

I understand the basics of ignition mapping both 2D & 3D, essentially our bikes have either a very simple centrifugal advance (up to 82) or a simple 2D advance map inside the ignitor (post 82).

Does anyone know or done any testing on different units?

I believe all the mechanical advances for similar engines are the same unit regardless of CC's (the difference here would probably be generated by spring strength changes).
Presumably the electronic maps have some differences but again I believe they would be similar.

The Dyna 2000 has 4 base maps for an 8V, 4 for a 16v, whilst there are differences the curves are reasonably similar & seen as basically "safe" for most engines. I should scan & post the curves from the instruction book for info/discussion point I guess....

Has anyone done any measurements whilst running or know what the differences between the maps are? Total advance on the 8v engines should be the same, Total advance on the 16v engines should be slightly more but likewise the same.

One upgrade in the car world when using different carbs & cams is to decrease the spring strength on the centrifugal advance so that the engine pulls total advance sooner, sometime stops or cams are filed down to adjust the total advance that can be reached, anyone try anything like that?

Dan

 

[Dave8338]

The vacuum advance on my Mustang is blocked off all together inorder no to go "beyond" the point of no return ie. too much.

Higher compression - less advance BTDC
Lower compression - more advance BTDC
That about sums it up... All in an attempt to avoid pre ignition.

 

[rapidray]

Also, you got it backwards Dan. The 8 valvers need, & can use, more advance without detonation than the 16 valvers. This is because of the larger combustion chamber volumes of the 8 valvers. You need the spark sooner on them to get more complete burn. Emissions wise, that is the reason most, if not all, new engine designs have small combustion chambers. Ray.

 

[salty_monk]

Ah.. thanks Ray.

So anyone know the differences between the advance curves on similar engines with different CC's?

Dan

 

[GregT]

To the best of my knowledge you're right about the curve being very simple on these motors - both 8 & 16v.
To all intents it's a straight line with max advance being reached about 2500 RPM
It's quite possible that a programmable curve & plenty of dyno time would see an improvement - if the motors were being made now that's what would be on them for emissions.
It's arguable whether anything better is needed for a race application - so long as you can start it, fixed ignition set to the max advance possible without detonation - and good fuel - seems to work quite well.
In my experience Ray, 2v/cylinder heads often do need more advance, yes partly as a result of poor chamber shape - high domes - but often due to a lack of turbulence. 4v heads pretty well always have better turbulence hence burn better (Honda 750/900/1100 family excepted)
For street use with the ever dropping quality of fuel, a stand - alone det sensor would be a nice plug - in

Greg T

 

[trippivot]

2 valve GS engines run best at 39 degrees MAXIMUM total advance under power load, but the stock set up starts at 16 degrees setting for a 1000rpm idle and only goes to 33 after 2000rpm. I say mechanical advance is linear not curved.
16 valve engines run great at 32 total advance. linear-rpm driven advance as well

My understanding is = it is the combustion chamber efficiency not the C.C. volume that effects total advance.

the flatter top piston joined with a flatter inner head design allows for a quicker flame front compared to a hemi bowl dome and curved piston crown that the flame front has to go around instead of straight across.

as for stock electronic advancing type"brains" on aircooled carbureted engines are basicly rpm contoled and linear too. even if the graph line is bell curved the event is not dynamicly curved like a 3-D system. Dyna units are rpm driven only so settings are aggressive or lazy

when there is a throttle position sensor, rpm sensor, speed sensor, knock sensor,temperature, etc...then -that is where 3-D curves and real time resetting occour and are necessary to keep the power /torque on tap conjoined to fuel delivery systems.

 

[posplayr]

One upgrade in the car world when using different carbs & cams is to decrease the spring strength on the centrifugal advance so that the engine pulls total advance sooner, sometime stops or cams are filed down to adjust the total advance that can be reached, anyone try anything like that?

Dan

Dan,
Funny you should ask I was relating the following story to RustyBronco via PM just recently. IIRC the stock GS1100E numbers are nearly the same as I modified my 64 Chevelle w/ 327 almost 35 years ago.

Bill and I need to be more careful. I never realized how advanced these motors are actually running STOCK. Well now that I say that I should have know because the timing for the GS1100ED stock is where the hot rodded ignition timing in the 60's-70's was for a small block Chevy

We would modify stock ignition from 4 deg initial and 28 deg mechanical for 32 total
to 12 initial and 20 deg mechanical for the same 32 total. really woke up the engine. That is essentially where the GS1100E is set from the factory.